The mechanisms underlying hormonal gene expression and secretion are extremely complex, and key regulatory events are often separated in time. The understanding of these dynamic processes has been facilitated in part by the use of reverse hemolytic plaque assays which enable multiple measurements of hormone release from the same living endocrine cells. Progress in this area has been hindered by the lack of a method suitable for making dynamic measurements of gene expression. To monitor molecular events, the principal investigator has modified conventional luciferase reporter technology to enable "real-time" measurement of gene expression in individual endocrine cells. With this tool and the plaque assay, the principal investigator is able to pursue new levels of questions about gene expression and hormone secretion that were not addressable previously because of technical constraints. The specific aims of the current proposal are to 1) optimize and complete validation of the system for real-time measurement of gene expression; 2) establish the temporal relationship between gene expression and secretion in normal pituitary cells; and 3) elucidate the mechanisms governing the transdifferentiation of GH and PRL secreting cells. The results of these studies may greatly expand the fundamental knowledge about the relationship between hormonal gene expression and secretion in normal cells. This information may be critical to gaining an understanding about aberrant mechanisms leading to pathophysiologic states of hormone secretion.